Britain's Stonehenge once had a long-lost twin just a stone's throw away from the prehistoric monument, archaeologists announced Thursday.

The discovery, made completely without digging, suggests that now solitary Stonehenge may have been surrounded by "satellite Stonehenges," archaeologists say.

"This finding is remarkable," said survey-team leader Vince Gaffney, an archaeologist the University of Birmingham in the U.K. "It will completely change the way we think about the landscape around Stonehenge."

Using the latest geophysical imaging technology, Gaffney's team captured digital impressions of the now buried remains of the newfound henge formation, just over half a mile (900 meters) from its world-famous neighbor.

Measuring 82 feet (25 meters) wide, the circular feature had a segmented ditch dotted with 20 or so large holes—suspected to have been postholes for a timber, rather than stone, circle, the team says. (Also see "Wooden 'Stonehenge' Emerges From Prehistoric Ohio.")

The circle's estimated date of 2,500 to 2,200 B.C. suggests "it was operating when Stonehenge was in its final and most dramatic form," Gaffney told National Geographic News (interactive Stonehenge time line).

Currently the leading view is that the immediate area around Stonehenge was a sacred, off-limits area where only a select few, such as high priests or nobility, were allowed. (See "Stonehenge 'Hedge' Found, Shielded Secret Rituals?")

"If [the newfound henge] was there at the same time, it demonstrates there was massively more activity going on in the landscape adjacent to Stonehenge," Gaffney said.

That isn't to say Stonehenge was open to anybody, he added, "but we are saying there seems to be more activity inside that boundary.

"Stonehenge," he added, "is one of the most studied monuments on Earth but this demonstrates that there is still much more to be found."

The team suggests the supposed wooden henge, like Stonehenge, performed an important ceremonial role for ancient Britons who gathered at the summer and winter solstices to mark the passing year with sacred rituals.

Also like Stonehenge, the now vanished henge is oriented toward the solstice sunrise with entrances to the northeast and southwest.

"This new monument is part of a growing body of evidence which shows how important the summer and winter solstices were to the ancient peoples who built Stonehenge," said Amanda Chadburn, as archaeologist with English Heritage, the government agency that manages the Stonehenge World Heritage site.